Re: Most physically powerful boxer...
It's really impossible to say, because of a variety of factors.
Firstly, manhandling someone in a clinch is as much a test of balance and leverage as pure strength. Punching power is not a good indicator of strength at all.
By definition, a big muscle is a strong muscle, but other factors like tendon strength and natural leverages play a big role.
If all the champions of say, the heavyweight division, were to perform a certain series of lifts such as the squat, bench press, military press, deadlift and barbell row then maybe we'd get a clearer picture of their raw strength in relation to each other.
Even here though, the tall guys like Bowe, Lewis, Carnera etc. with long arms, backs and legs would be at a disadvantage compared to the shorter guys like Marciano, Frazier etc.
In that case, we'd have to include some other strength-related movements like a farmer's walk or something like that. It's really impossible to accurately guage raw strength from one individual to another.
I have heard of some boxers performing rather remarkable feats of strength though. Louie Espinoza was said to be able to bench press 320 lbs. as a featherweight.
Evander Holyfield regularly repped out with 360 lbs. on the bench press and Tyson was said to bench press 240 lbs. as a 14 year old.
Paul Anderson has already been mentioned, but Franco Columbo (2x Mr. Olympia) dabbled in boxing as well, and he could perform a 750 lb. deadlift.